Mass spectrometry



Sept. 17, 1957 L. G. HALL ETAL MASS SPECTROMETRY Filed July 11, 1955 INVENTORS. LAWRENCE 6. HALL BY CHARL 3 E ROBINSON m mdem ATTORNE Patented Sept. 17, 1%57 MASS srnc'rnos arnr Lawrence G. Hall, West Covina, and Charles F. Robinson, Pasadena, (lalih, assignors, by mesne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Application July 11, 1955, Serial No. 521,137

7 Claims. (Cl. Za -41.9)

This invention relates to mass spectrometry and particularly to improvements in mass spectrometers of the type in which an electrical field is formed within an analyzer region by means of a plurality of electrodes essentially defining the boundaries of such region. Cycloidal and ion resonance mass spectrometers are examples of this type of instrument. To facilitate understanding of the invention it is described with relation to its application to a cycloidal mass spectrometer, but is in no way limited to this particular type of instrument.

The principle of operation of a cycloidal or so-called cross-field mass spectrometer is as follows: If a charged particle is introduced into a uniform magnetic field it will move in a circular path to return to its point of origin. This is true regardless of the mass of the particle, with particles of heavy mass traveling in circles of greater radius than particles of lighter mass, but in each instance returning to the point of origin. If a uniform electric field is imposed across the space defined by the magnetic field and normal thereto, the ions pursue a path which may be considered as rigorously circular in a coordinate system moving with uniform velocity. The movement of the coordinate system is a function of the ratio of the electrical and magnetic field strengths. If ions of a particular mass are introduced into such a system they will complete one turn of their circular motion in a time which depends directly on the mass of the particle, and if the electric field strength is uniform so that the coordinate systems corresponding to each particle move at the same velocity, the particles will converge to a series of rigorous point foci after an integral number of turns in the magnetic field and regardless of their velocity or direction of travel at the moment of introduction into the field.

Since the time required for ions to complete one turn of their circular motion depends directly on mass and since under the conditions of uniform field strength specified the rate of motion of the coordinate system is invariant to the mass of particles involved, the focal point of the heavy particles will be displaced farther from the point of origin than the focal point of the lighter particles. This is the basic concept employed in the cycloidal mass spectrometer.

To form the electrical field with the uniformity desired, a plurality of electrodes are arranged to define the analyzer region in which mass analysis is accomplished and potentials are applied to these electrodes to distribute the field across the analyzer region as required. Without discussing the operation of an ion resonance mass spectrometer a plurality of electrodes are similarly employed to provide across a region of ion analysis a field of desired configuration.

We have now found that the operation of instruments of this type is greatly improved if heat is supplied to the electrodes defining the analyzer region and in such quantities as to appreciably raise the temperature of these electrodes above that reached in conventional operation. In other Words, a normal operation of a mass spectrometer, for example a cycloidal mass spectrometer, results in the generation of a certain amount of heat principally at the filament of the ion source, which raises the temperature of various parts of the instrument above ambient. However, this inherent heating is far from uniform throughout the instrument, and we have found advantages resulting from further heating not only to obtain a degree of uniformity but to elevate the temperature of the field forming electrodes significantly above that inherently achieved.

Our invention contemplates in a mass spectrometer the combination comprising an envelope, means forming an ion source within the envelope, means forming an analyzer within the envelope, the analyzer being defined as that section within the envelope in which ion separation is accomplished, means causing ions to travel from the source into the analyzer, collector means disposed to collect ions traversing the analyzer, and means operable to supply heat to the analyzer during operation of the instrument. The application of heat to the type of mass spectrometer here under consideration and during the actual operation of the instrument is not to be confused with the classical bake-out techniques used for preconditioning mass spectrometers of various type prior to sale or use.

It has also been found that good results are achieved in the practice of the invention if the desired auxiliary heat is supplied by a plurality of heat sources actually disposed within the envelope whereby the heat may be applied directly to the electrode array itself. By applying this heat to certain of the electrodes and by interconnecting these electrodes by electrical insulation of comparatively high heat conductivity, a very satisfactory temperature profile can be obtained.

The invention will be clearly understood from the following detailed description thereof as applied in cycloidal mass spectrometry and taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a longitudinal sectional elevation through a cycloidal focusing mass spectrometer;

Fig. 2 is a schematic illustration of the ion source as though taken on the line 2-2 of Fig. 1; and

Fig. 3 is a sectional elevation taken on the line 3-3 of Fig. 2.

The mass spectrometer shown in Fig. 1 comprises an evacuable envelope 5 supported from a flanged socket 4 and provided with conduit means 6 for connection to an evacuating system (not shown). A sample inlet is brought through an end of the envelope. A plurality of electrodes 9, iii, 11, i2, 13 and 14 are supported in the envelope between plates 8 and 15.

The electrode structure is supported by the median electrode 12 which is mounted to a mounting flange l6 and electrically insulated therefrom by an insulating spacer 16A. The mounting flange is rigidly supported by the socket 4. To reduce heat loss from the electrode sys tem, electrode 12 is preferably hollowed out intermediate the mounting flange and the analyzer so that this section of the electrode comprises two spaced bars extending from the flange to the analyzer.

The several electrodes are supported as illustrated by a series of pins i7, 18, 19, 20, 21, 22 and as supported are spaced and insulated from each other by insulators 23. The electrode structure defines an analyzer chamber 24 having an inlet slit 25 and resolving slit 26 spaced from each other on a common or so-called focal plane. An ion source 28 is supported adjacent the analyzer 24 and includes a terminal accelerating electrode 29 defining the above-mentioned inlet slit 25.

As shown schematically in Fig. 2, the ion source includes an ionizing region 30, electron gun 31, an electron target 32, a repeller electrode 33, .a first accelerating electrode 34 having an aperture 35 and the electrode 29 defining the inlet aperture 25 to the analyzer. The above elements of the ion source are arranged so that molecule introduced into the chamber 30 through the inlet 7 (see Fig. 1) are ionized by an electron beam 36 shown in dotted line traversing the region between the electron gun 31 and the target 32. Under the influence of thepotential between the repeller electrode 33 and the accelerating electrodes 34 and 29, the ions are propelled through aperture'25 into the cycloidal analyzer.

Appropriate potentials are impressed on the several electrodes 9, 16, 11, 12, 13 and 14 by means of actinventional voltage supply network (not shown). A mass spectrum can be scanned by varying the field strength within the analyzer without altering the relative potentials established between the electrodes, thereby causing difierent ion trajectories to focus at the resolving aperture 26. Means for applying such potentials to the electrodes and of varying the same to accomplish spectrum scan are well known in the art and are described in many prior patents and applications, as for example in copending application Serial No. 497,097, filed by Charles FhRobinson on March 28, 1955, which is directed specifically to cycloidal mass spectrometry.

In accordance with preferred practice of the present invention, heat is supplied directly to plate 8, to plate 15 and to the ion source so as to establish throughout the electrode array which defines the analyzer a synthetic elevated temperature of comparatively uniform distribution. A heat source 46) is shown arranged to supply heat to the plate 8, the source being provided with a shield 41 illustrative of means for insming adequate heat transfer between the source and the plate. A second source of heat 42 is similarly associated with plate 15 and is provided with a shield o-r hood 43 to further insure suitable heat distribution to the electrode array. A third source of heat is preferably applied to the ion source.

An ion source of the type illustrated produces a certain mnount of heat, particularly at the electron gun, but We have found that for best results in practicing the invention a separate source of heat can be applied directly to the ion source. This separate heat source generally adds only sufficient heat to compensate for variations in the temperature of the source, and to this end is preferably located in the neighborhood of the electron gun. A preferred arrangement is shovm in Fig. 3, which is an enlarged View of the electron gun taken on the line 3"?) of Fig. 2. The gun comprises an electron emitting filament 44 supported between a post 45 and a flexible hinge 46 in turn mounted on the end of a second post 47, the posts 45 and 47 furnishing conduit for electrical leads to supply power to the filament 44. A heater element 48 is disposed adjacent electron emitting filament 44 and is supported in that position between a third post 49 and a projection 50 extending inwardly from the post 47. To further enable control of the source temperature, a copper wire 53 of a desired dimension i connected from the source block to the base connections. This wire provides a heat loss to dissipate excess heat and to maintain the nominal temperature of the source substantially the same as the field plates.

The entire spectrometer tube is immersed in a magnetic field oriented normal to the electrical field developed by the electrode array. One pole 54 of magnet means (not otherwise shown) for producing this magnetic field is illustrated.

The following table of the temperatures conventionally encountered in this type of instrument and those artificially induced in thepractice of the. invention will show the magnitude of the temperature diiferentials considered.

It will be seen that the temperatures in which the electrodes are maintained in accordance with the invention are of a different order of magnitude than those inherently produced. The higher temperatures of the electrodes forming the analyzer result in a marked decrease in interference of one material on another within the analyzer and a consequent higher sensitivity at the collector. This reduced interference is thought to be due in part to the fact that the electrodes become less tenable depositories for reacting materials andhence surface interferences are minimized. The invention is not intendedto be in any way limited to this explanation as it is obviously not a complete explanation of the effects of the higher temperature.

Optimum heat distnibution throughout the analyzer is achieved if .the insulating spacers between the several electrodes are of relatively high heat conductivity. For this reason synthetic sapphires are preferred over glass insulators because they exhibit better electrical insulation by a factor of about ten at high temperature, and at the same time better thermal conductivity by a similar factor.

As made clear at the beginning of this disclosure, the invention is not limited to cyclo idal mass spectrometry but is directed to any form of mas spectrometer in which an analyzer'region is defined by a plurality of electrodes producing the electrical field required in the analyzer region. Nor is the invention limited to application of such heat in the manner specifically illustrated and described, although application internally of the spectrometer and particularly as illustrated is preferred in the particular type of cycloidal mas spectrometer as illustrated.

We claim:

1. In a mass spectrometer the combination comprising an envelope, means forming an ion source within the envelope, means forming an analyzer within the envelope, electrodes disposed in the analyzer to develop an electric field therein, means causing ions to travel from the source into the analyzer, collector means disposed to collect ions traversing the analyzer, and a plurality of separate heater means disposed at spaced points within the envelope and operable to heat the analyzer during operation of the instrument.

2. In a mass spectrometer the combination comprising an envelope, means forming an ion source Within the envelope, a plurality of electrodes defining an analyzer within the envelope, means including the electrodes for producing an electrical field across the analyzer, means producing a magnetic field across the analyzer normal to the electrical field, collector means disposed to collect ions traversing the analyzer, andmeans disposed exteriorly of the ion source and within the envelope for supplying heat to the electrodes during operation of the instrument.

3..In a mas spectrometer thecombination comprising an envelope, means forming an ion source within the envelope and including an electron emissive filament, a plurality of electrodes defining an analyzer within the envelope, means including the electrodes for producing an electrical field across the analyzer, means producing a magnetic field across the analyzer normal to the electrical field, collector means disposed to collect ions traversing the analyzer, first heater means in addition to the electron emissive filament for supplying heat to the ion source, and second heater means .for supplying heatto the electrodes which define the analyzer during operation of the instrument.

4. In amass spectrometerthe'combination comprising an envelope, means forming an ion source Within the envelope, a plurality of electrodes defining an analyzer Within the envelope, means including the electrodes for producing an electrical field across the analyzer, means producing a magnetic field a cross the analyzer normal to the electrical field, collector means disposed to collect ions traversing the analyzer, and a plurality of separate heater means disposed at spaced points within the envelope and outside the ion source for supplying heat to the electrodes which define the analyzer during operation of the instrument.

5. In a mass spectrometer the combination comprising an envelope, means forming an ion source within the envelope, a plurality of electrodes defining an analyzer within the envelope, electrical insulators of relatively high thermal conductivity disposed between and separating the electrodes, means including the electrodes for producing an electrical field across the analyzer, means producing a magnetic field across the analyzer normal to the electrical field, collector means disposed to collect ions traversing the analyzer, and means for supplying heat to certain of the electrodes during operation of the instrument whereby the other electrodes are heated by thermal conduction through the insulators.

6. Apparatus according to claim 5 wherein the insw lators are synthetic sapphires.

7. In a mass spectrometer the combination compris ing an envelope, means forming an ion source within the envelope and including an electron emissive filament, a plurality of electrodes defining an analyzer Within the envelope and supported between top and bottom plates, electrical insulators of relatively high thermal conductivity disposed between and separating the electrodes from each other and from the plates, means including the electrodes for producing a magnetic field across the analyzer normal to the electrical field, collector means disposed to collect ions traversing the analyzer, separate heater means at each of the top and bottom plates for supplying heat to the electrodes during operation of the instrument through thermal conduction through the insulators, and a separate heat source disposed in the ion source adjacent the filament.

References Cited in the file of this patent UNITED STATES PATENTS 

